1. Technical Field of the Invention
The present invention relates to rotors for automotive alternators that are used in, for example, passenger cars and trucks.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2002-354720, an English equivalent of which is U.S. Pat. No. 6,476,535 B1, discloses an automotive alternator. This alternator is designed to increase an output of the alternator by reducing leakage magnetic flux that flows between adjacent magnetic pole claws of a rotor via teeth of a stator core when the magnetic pole claws are at a predetermined relative position to the teeth of the stator core in the alternator.
Specifically, in the alternator, each of the magnetic pole claws of the rotor has a pair of chamfered portions formed respectively at opposite circumferential ends of a root shoulder part of the magnetic pole claw. Each of the chamfered portions is made up of a surface that includes: a point B on a line from which a taper formed on an outer peripheral surface of the root shoulder part starts; a point C on a skew slanted portion of the magnetic pole claw; and a point A on a circumferential end surface of the magnetic pole claw. Consequently, when the chamfered portions of the magnetic pole claws of the rotor are positioned so as to radially overlap distal ends of the teeth of the stator core, air gaps formed between the magnetic pole claws and the teeth of the stator core are increased by the chamfered portions in the overlapping regions. Further, with the increase in the air gaps, the magnetic reluctance between the magnetic pole claws and the teeth of the stator core is accordingly increased in the overlapping regions, thereby reducing the leakage magnetic flux that flows between the adjacent magnetic pole claws of the rotor via the teeth of the stator core. As a result, the effective magnetic flux in the alternator is accordingly increased, thereby increasing the output of the alternator.
However, in the above alternator, to reduce the leakage magnetic flux, it is necessary for the chamfered portions to exist on the magnetic flux path when the adjacent magnetic pole claws of the rotor radially overlap the distal ends of the teeth of the stator core. On the other hand, for each of the chamfered portions, the boundary line (i.e., the line connecting the points B and C) between the chamfered portion and the outer peripheral surface of the magnetic pole claw extends obliquely with respect to circumferential ends of the teeth of the stator core. Therefore, at those areas of the chamfered portion which are close to the boundary line, the radial distance between the magnetic pole claw and the teeth of the stator core is short. In contrast, at those areas of the chamfered portion which are distant from the boundary line, the radial distance between the magnetic pole claw and the teeth of the stator core is long. Consequently, when the radial distance is increased for reducing the leakage magnetic flux at those areas of the chamfered portion which are close to the boundary line, the radial distance may become excessively long at those areas of the chamfered portion which are distant from the boundary line, thereby reducing the effective magnetic flux. As a result, it may be difficult to achieve the desired effect of increasing the output of the alternator by reducing the leakage magnetic flux.
Moreover, with the above-described configuration of the chamfered portions, there still remains the problem that the chamfered portions function as a fan to generate wind noise during rotation of the rotor. In addition, it may be difficult to minimize the manufacturing cost of the alternator.